On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

140 Chapter 3. Solving the Flavor Problem in Strongly Coupled Theories
(a) (b) (c) (d)
(e) (f)
Figure 3.13: In the first row, the leading diagrams for the single and pair production
of axigluon or scalar octet resonances are shown for gluon initial states. Coiled lines
represent gluons, straight lines quarks and dashed lines can be either an axigluon KK
mode or a scalar octet. The diagrams (b) and (c) exist also for an axigluon (octet) in
the loop and a octet (axigluon) final state. The second line shows the favored decay
channels for the pair production, with the straight lines in (e) denoting top quarks in
case of a neutral octet and top-bottom pairs if the octets are charged.
from gluon initial states [193]. However, even though they are color charged, the new
scalars are electroweak doublets and can therefore only be produced from gluon initial
states via loop suppressed gluon-gluon fusion, or in pairs, see Figure 3.13.
The same holds true for the axigluon. From the second line of (3.84) it is clear that
a GGA vertex is forbidden, as well as any vertex with an odd number of axigluons if
θ = 45 ◦ , that is, if the new resonance couples purely axial. This is a result of parity
conservation, since the axigluon must have negative parity. As a consequence the
production channels from gluon initial states for a heavy axigluon resonance are also
given by the diagrams in the first row of Figure 3.13.
Regarding the decays one can say, that the color octet will decay primarily into tops,
because its couplings are proportional to the Yukawa couplings. This is also true for
the axigluon in the RS model, because it will be strongly IR localized and the RS-GIM
mechanism suppresses couplings to the light flavors. Both will therefore also evade
the usual dijet bounds, because a top pair final state will not look like two jets in the
detector and is often not reconstructed in these analyses. Possible signatures would
be seen in resonance searches in top pair final states for the gluon-fusion production
channel, or in four top final states if the resonances are pair-produced at tree-level.
There is also the diagram (f) in Figure 3.13, but this will not produce a narrow resonance
in the dijet spectrum. Scalar octets can also radiate off an electroweak gauge
boson before they decay, which is however suppressed if the decay in top pairs is kinematically
accessible, see [200, Fig.4] and the analysis therein.
In order to estimate whether the extended RS model is in conflict with the direct
detection limits we will concentrate on the axigluon KK mode. For this resonance the